In response to the demand for reduction in power consumption of semiconductor devices, studies are being made on methods for controlling the power supply voltage. The power supply voltages of semiconductor devices influence the operation speeds of the semiconductor devices, so that the voltages are reduced while the characteristics of the transistors are taken into consideration. However, with miniaturization of the manufacturing process, the causes of variations in transistors increase, and the characteristics of the transistors fluctuate greatly.
Therefore, in consideration of manufacturing variations, temperature fluctuations and the like, a method called AVS (adaptive voltage scaling) is adopted which lowers a power supply voltage within a range where a target circuit operates normally to reduce power consumption. However, the method cannot cope with a case where abrupt reduction in power supply voltage occurs due to droop of a current or the like, and cannot reduce power consumption sufficiently.